1. Prior Art
Modern steel products are diverse and are required to meet increasingly stringent quality standards. Under these circumstances, the control of impurity elements in steel products is an important task in the steel industry. In almost all instances, this has been done by sampling a portion of the molten steel to be analyzed and measuring the concentrations of impurity elements in the melt with analytical instruments. However, this method lacks rapidity required in commercial operations.
A method for measuring the concentrations of impurity elements in molten iron with rapidity has recently been proposed (see Japanese Kokai No. 142455/1986). This method uses a probe comprising a solid electrolyte having on its surface a coating layer comprising an oxide of an impurity element to be analyzed or a composite thereof with another oxide. The probe is immersed in molten iron and the oxygen partial pressure created by the reacting of equilibrium between the impurity element of interest and an oxide thereof is measured using the principle of an oxygen concentration electrochemical cell. The probe designed to determine the concentration of an impurity element of interest by this method is hereunder referred to as a chemical potential sensor.
2. Problems of the Prior Art
The sensor described in Japanese Kokai No. 142455/1986 is essentially a chemical sensor utilizing the reaction of chemical equilibrium between impurity elements in molten iron and oxygen at high temperatures. Thermodynamically, no ideal reaction should occur in such a way that only the element to be analyzed will interact with oxygen without being affected by other elements. According to repeated experimentation by the present inventors, measurements of the Si concentration of molten iron are affected by other high-content elements such as C, Al and Ti or those elements which have greater affinity for oxygen. It has therefore become clear that in order to eliminate these defects, some compensation is imperative.
This is technologically a serious problem to the chemical potential sensor under consideration. In the prior art, correct estimation of the concentration of a particular impurity element has been difficult unless the effects of unwanted elements are compensated for by comparison against calibration curves or the concentrations of such elements are already known by measurements with other analytical instruments. In the former case, much data is necessary to construct the calibration curves and the values estimated with the sensor are not reliable if they are outside the applicable range of the calibration curves. In the latter case, the need to use special analytical instruments slows down the analytical procedure. Furthermore, such analytical instruments themselves are capable of determining the concentration of the impurity element and render the use of the sensor meaningless.
The probe of the present invention solves all of the problems described above. It basically consists of two or three sensors, the first sensor comprising a solid electrolyte having oxygen ion conductivity and which has formed on it a coating layer that will provide a constant value of the activity (a.sub.M.sbsb.1) of an oxide of an impurity element (M.sub.1) to be measured in molten iron, and the second and third sensors being designed for measuring the activity of free oxygen (a.sub.o) and the content of carbon, respectively. This probe system not only improves the precision of measurements significantly but also expands the range of applicable conditions for measurement.
The present invention provides a composite probe for direct measurement of the concentration of a particular impurity element in molten iron. The probe consists of:
at least one first sensor for measuring the concentration, NM, of an impurity element M in molten iron, said first sensor being composed of a solid electrolyte having oxygen ion conductivity, a standard electrode provided in the interior of said solid electrolyte and which produces a known value of oxygen partial pressure, and a counter electrode provided on the exterior of said solid electrolyte which determines a reference potential, said solid electrolyte being coated with a substance that provides a constant value of the activity of an oxide of the impurity element M in molten iron; at least one of PA1 a second sensor comprising a solid electrolyte having oxygen ion conductivity and which measures the activity, a.sub.o, of free oxygen; and PA1 a third sensor for measuring the content of carbon in the molten iron, PA1 said first, second and/or third sensors being assembled in the single probe .